This invention relates to a novel CRT (cathode-ray tube) and particularly to a CRT having an internal electrically-conductive stripe or patch and to a method for preparing that CRT.
A CRT usually includes an evacuated glass envelope, an electrically-conductive funnel coating or layer on the inside wall of the envelope and a metal anode button sealed through the wall of the envelope and in electrical contact with the funnel coating. The funnel coating is used to carry a high potential that is applied at the anode button. Some CRT types include a metallized luminescent viewing screen supported on the inner surface of the faceplate of the envelope, and an apertured mask assembly closely spaced from the viewing screen and supported on several metal studs imbedded in the wall of the envelope.
The proper operation of the CRT requires that there be electrical continuity between the viewing screen and the anode button. When there is a discontinuity, electrical charges build up on the screen causing erratic operation of the CRT. Usually the break in continuity is found on the inside surface of the envelope adjacent the studs or the anode button. In order to insure electrical continuity, it has been the practice to apply an electrically-conductive coating in the form of a stripe or patch to at least one of the studs and/or the anode button. This has been done by brush painting or otherwise coating a stripe or patch of a composition which, after drying and heating, to about 400.degree. C., forms a chemically-stable, electrically-conductive stripe or patch across the area of interest. Most prior stripes or patches consist essentially of graphite with an alkali silicate binder, similar in character to the funnel coating. Such prior coating compositions have several disadvantages for these applications. They are relatively slow to dry. After they dry, they are not sufficiently insoluble in water to withstand subsequent processing with aqueous media.